Add GPG master and signing key for arch-boxes

The key is used for signing the releases, so the users can be sure the
images on the mirrors haven't been modified. arch-boxes has been tweaked
to use the key in this MR[1].

[1] arch-boxes!176

misc/keys/README.md

 # Keys
 
-This directory contains the GPG master and signing key used by [Renovate](https://github.com/renovatebot/renovate) for signing its commits.
+This directory contains the GPG master and signing keys used by the following projects:
+- [Renovate](https://gitlab.archlinux.org/archlinux/renovate/renovate) for signing commits
+- [arch-boxes](https://gitlab.archlinux.org/archlinux/arch-boxes) for signing releases
 
-The keys were generated with the following commands:
+The Renonvate keys were generated with the following commands:
 ```sh
 $ export GNUPGHOME="$(mktemp -d)"
 $ gpg --quick-generate-key 'renovate <renovate@archlinux.org>' rsa4096 cert never
@@ -13,4 +15,15 @@ $ gpg --armor --export-secret-subkeys "${key_id}"
 $ rm -r "${GNUPGHOME}"
 ```
 
-The exported signing key has been added as a GitLab CI/CD variable to the [renovate project](https://gitlab.archlinux.org/archlinux/renovate/renovate). The master and and a copy of the signing key are stored in the [`renovate.asc`](renovate.asc) file.
+The arch-boxes keys were generated with the following commands:
+```sh
+$ export GNUPGHOME="$(mktemp -d)"
+$ gpg --quick-generate-key 'arch-boxes <arch-boxes@archlinux.org>' ed25519 cert never
+$ key_id="$(gpg --with-colons --list-keys arch-boxes@archlinux.org | awk -F : '$1 == "fpr" {print $10;exit}')"
+$ gpg --quick-add-key "${key_id}" ed25519 sign 5y
+$ gpg --armor --export-secret-keys "${key_id}"
+$ gpg --armor --export-secret-subkeys "${key_id}"
+$ rm -r "${GNUPGHOME}"
+```
+
+The exported signing keys have been added as GitLab CI/CD variables to the projects. The master keys and a copy of the signing keys are stored in the [`renovate.asc`](renovate.asc) and [`arch-boxes.asc`](arch-boxes.asc) file.

misc/keys/arch-boxes.asc

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